The present invention relates generally to avalanche photodiode (APD) gain control circuits, and more specifically to a method using an adaptive power supply for avalanche photodiodes that provides a means for determining an optimum bias voltage for the APD.
Avalanche photodiodes are used in optical receivers for converting an optical signal into an electrical signal. The electrical signal output from the APD is coupled to an amplifier for amplification. One of the most important parameters of an APD is the reverse bias voltage associated with breakdown. When operated below the reverse breakdown voltage, increases in reverse bias results in amplification. This is the region of normal APD operation. However at the breakdown voltage dark currents increase exponentially, causing the receiver to be saturated with noise and possibly damaging or destroying the APD.
Therefore the APD generally is biased close to its breakdown voltage to achieve maximum sensitivity. Typically the bias voltage for an APD is set at a specified number of volts, such as five volts, below the specified breakdown voltage. However since each APD is different, the breakdown voltage of each APD is different. This requires different bias voltages from APD to APD.
A measure of the reliability of an APD is the ability to pass optical digital data signals with a virtually zero bit error rate (BER)--for SONET systems the virtually zero BER is specified to be 10.sup.-10 or less. For different optical power levels, the bias voltage at which the bit error rate becomes non-zero differs. Especially at larger voltages and/or larger optical power levels the APD generates more noise which causes the bit error rate to be non-zero.
What is desired is a method using an adaptive power supply for an avalanche photodiode for determining an optimum bias voltage for the avalanche photodiode.